FIG. 1 depicts a conventional computer system 100 having a conventional LCD panel display 10. A backlight inverter 20 is provided to drive one or more cold cathode fluorescent lamps (CCFLs) 22 and/or 24, includes a transformer 26 and controller 28, as is well understood in the art. A conventional computer system provides a LCD panel brightness level according to the command signal from the user, such as from the keyboard input or a potentiometer in the computer. The brightness of the LCD panel is fixed once the manual setup is set. The amount of power delivered to the backlight system is therefore fixed regardless the ambient brightness variations. Traditionally, users do not take the advantage of reducing the power level when the ambient light is reduced. To achieve a better utilization of battery power for appropriate LCD panel brightness while ambient light is changing, it is essential to implement auto-brightness control means to extend the battery run time for portable electronics. A light sensor 30 is provided to generate a signal indicative of the ambient light around the panel 10. The signal is fed to the backlight inverter 20 to adjust the amount of power delivered to the CCFL(s).
In a computer, the system may also include a system CPU 40 and line memory 50. The panel 10 may include a thin film transistor array (LCD) a scanner to synchronize operation of the LCD and a video data input module 16 and 18 to receive video data from line memory 50. These components are well understood in the art.
FIG. 2 depicts a conventional control system 200. In the conventional system, the panel brightness is controlled by comparing the current flowing through the CCFLs with the signal from the ambient light sensor output. The feedback signal in this control system is the sensed current flowing through the CCFL(s). This implementation represents an open-loop control with respect to the ambient brightness and the LCD brightness. One challenge is the LCD panel brightness varies even with the same amount of current flowing through the CCFL(s). Panel brightness can also vary on the manufacturing of the LCD panel including the material, thin-film-transistor technology, mechanical arrangement and the structure of the backlight module. This implementation is impractical and does not satisfy general requirement. As is known in the art, the display brightness lowers by more than half of the default brightness under cold temperature conditions with the same amount of current flowing through the CCFL(s). For example, the brightness of a LCD panel under room ambient condition of 350 lum/m^2; reduces to 120 lum/m^2 under −30 degree C. ambient condition. As a result, the brightness output from a display does not satisfy the users' requirement even under the same ambient light condition. Another example is the LCD display in a navigation system (global position system) in a car. Under low-temperature ambient, such ambient-sensor control system implementation for LCD in a car, does not produce sufficient light as needed or even fail to produce light output. Thus, this control system cannot sufficiently provide closed-loop feedback information to more accurately control the brightness of the panel. A target panel brightness signal is the desired output which should be used as a control signal for the controller and the power supply. Such open-loop control relies greatly upon the efficacy between the amount of power to the CCFL(s) and the LCD brightness output. Therefore, there is a need for a control system where the LCD brightness responds panel brightness. Further, the perception of the comfort level of the brightness varies from one user to another. Therefore, it there is also a need to have a user-command input to the control system to set a desired default display brightness level to satisfy the users a comfort level of brightness.